The invention relates to a process for the production of moldings composed of a polyoxymethylene polymer via blow molding.
Blow molding processes are used in particular to produce hollow bodies. These can by way of example be storage containers or transport containers, for example for media which are gases, liquids, or solids. Examples of these storage vessels or containers are bottles, tanks, or the like.
Blow-molding processes usually melt a semifinished polymer product, generally pellets, in an extruder, and extrude the material to give tubing. The tubing is then molded in a blow mold to give the hollow body, by introducing a mandrel into the tubing and injecting a pressurized gas into the tubing via the mandrel, the result being that the tubing is pressed against the walls of the blow mold. The temperatures of the blow mold here are below the melting point of the polymer, and the polymer therefore solidifies in the mold.
A blow-molding process for the production of moldings composed of polyoxymethylene/polyurethane mixtures is known by way of example from EP-A 0 121 407. The polyurethane is added in order to improve the properties of the polyoxymethylene.
A process for the production of moldings via blow molding, from polyoxymethylene resins, is known by way of example from EP-A 0 568 308. The molecular structure of the polyoxymethylene copolymer used is in essence linear.
For the melting process, extruders with grooved feed sections have become established. These extruders with grooved feed sections are described by way of example in D. Boes et al., 30 Jahre Nutenextruder [30 years of grooved extruders], in Kunststoffe 80, 1990, No. 6, pages 659-664. In extruders of this type with a grooved feed section, the feed section determines the amount of polymer conveyed. The polymer pellets are compressed and conveyed in the feed section of extruders with a grooved feed section, and this results in high pressure. Melting of the pellets in the feed section is inhibited by cooling. However, a disadvantage with the usual method of operating a single-screw extruder with a grooved feed section when polyoxymethylene polymers are being processed to give blow moldings is that the moldings produced by the blow-molding process have poor mechanical properties, e.g. low tensile strain at break and reduced bursting pressure. Non-uniform wall thicknesses, and weld lines in the blow molding, can also occur.